You are looking for overrideredirect. In Tk overrideredirect does 2 things, it grabs all virtual desktops and removes Window Manager controls.

UPDATE: I just noticed an odd behavior with this script. With overrideredirect enabled, I can't get focus to the password entry widgets. Maybe you can fix it. :-)

#!/usr/bin/perl -w
use warnings;
use strict;
use Tk;
use Tk::LabEntry;

my $mw = MainWindow->new;
my $vh = $mw->vrootheight;
my $vw = $mw->vrootwidth;

# this is what grabs all virtual desktops
# and removes WM controls
$mw->overrideredirect(1);

# Note that the 'virtual window' height and width are $vh and $vw
# respectively, so we use those dimensions for our Canvas height 
# and width, and let the Canvas expand and fill in both x and y
# directions.
#
my $canvas = $mw->Canvas(
   -width => $vw, 
   -height => $vh, 
   -background =>'blue',
   #-takefocus =>0
   ); # so canvas dosn't take focus on tab press

$canvas->pack(-expand => 1, -fill => 'both');

#just for fun instead of an image
$canvas->createRectangle(100, 100, 150, 150, -fill => 'orange'); 

my %window;
my $ypos = 0;
foreach ('First', 'Last'){
     $window{$_}{'obj'} = $canvas->LabEntry(-label=> $_);
     $window{$_}{'obj'}->Subwidget('entry')->configure(
              -background=>'yellow',
          -textvariable => \$window{$_}{'data'},
          );
     $window{$_}{'obj'}->configure(-labelPack=>[-side=>'left']);
     
     $canvas->createWindow($vw/2, $vh/2 + $ypos,
          -window=> $window{$_}{'obj'});
     
     $ypos += 40;
}


$window{'Ok'}{'obj'} = 
    $canvas->Button(-text=> 'Ok',
               -command=> sub{ 
                       
                 foreach ('First', 'Last'){                       
               print  "$_ , $window{$_}{'data'}\n";                   
+ 
           
               }          
            exit;
        }
    );

$canvas->createWindow($vw/2, $vh/2 + $ypos + 50,
          -window=> $window{'Ok'}{'obj'});

$window{'First'}{'obj'}->focus;

# this is what grabs all virtual desktops
#$mw->overrideredirect(1);


MainLoop;
[download]